Organisms occupy different places in part because each species has a range of conditions under which it can grow and reproduce.  These conditions help define where and how an organism lives.

Tolerance

Every species has its own range of tolerance.

Tolerance
The ability to survive and reproduce under a range of environmental circumstances

When an environmental condition, such as temperature, extends in either direction beyond an organism’s optimum range, the organism experiences stress.  Why? Because it must expend more energy to maintain homeostasis, and so has less energy left for growth and reproduction.  Organisms have an upper and lower limit of tolerance for every environmental factor.  Beyond those limits, the organism cannot survive.  A species’ tolerance for environmental conditions, then, helps determine its “address” or habitat.

Habitat
The general place where an organism lives or can be found

Defining the Niche

Describing a species’ habitat tells only part of its story.  Ecologists also study a species’ ecological “occupation”—where and how it “makes a living.” This idea of occupation is encompassed in the idea of an organism’s niche. 

Niche
A full range of physical and biological conditions in which an organism lives and the way in which the organism uses those conditions

A niche describes not only what an organism does, but also how it interacts with biotic and abiotic factors in the environment.  A niche is the range of physical and biological conditions in which a species lives and the way the species obtains what it needs to survive and reproduce.  Understanding niches is important to understanding how organisms interact to form a community.

Resources and the Niche
The term resource can refer to any necessity of life, such as water, nutrients, light, food, or space.  For plants, resources can include sunlight, water, and soil nutrients—all of which are essential to survival.  For animals, resources can include nesting space, shelter, types of food, and places to feed.

Physical Aspects of the Niche
Part of an organism’s niche involves the abiotic factors it requires for survival.  Most amphibians, for example, lose and absorb water through their skin, so they must live in moist places.  If an area is too hot and dry, or too cold for too long, most amphibians cannot survive.

Biological Aspects of the Niche
Biological aspects of an organism’s niche involve the biotic factors it requires for survival.  When and how it reproduces, the food it eats, and the way in which it obtains that food are all examples of biological aspects of an organism’s niche.  Birds on Christmas Island, a small island in the Indian Ocean, for example, all live in the same habitat but they prey on fish of different sizes and feed in different places.  Thus, each species occupies a distinct niche.

What is a niche?
 

REVIEW & DO NOW Answer the following questions:
What is meant by a species' tolerance? What is meant by a species' habitat? What is meant by a species' niche? Review:  What is a community?	What kinds of resources do plants and animals need in their niche? What are the physical aspects of a niche? What are the biological aspects of a niche?

Competition
How does competition shape communities?
 

If you look at any community, you will probably find more than one kind of organism attempting to use various essential resources.  When organisms attempt to use the same limited ecological resource in the same place at the same time, competition occurs.  In a forest, for example, plant roots compete for water and nutrients in the soil.  Animals compete for resources such as food, mates, and places to live and raise their young.  Competition can occur both among members

of the same species (known as intraspecific competition) and between members of different species (known as interspecific competition).

The Competitive Exclusion Principle

Direct competition between different species almost always produces a winner and a loser—and the losing species dies out.  One series of experiments demonstrated this using two species of single-celled organisms.  When the species were grown in separate cultures under the same conditions, each survived, as shown in Figure 4–6.  But when both species were grown together in the same culture, one species outcompeted the other.  The less competitive species did not survive.

Experiments like this one, along with observations in nature, led to the discovery of an important ecological rule—the competitive exclusion principle.

The Competitive Exclusion Principle
No two species can occupy exactly the same niche in exactly the same habitat at exactly the same time.

If two species attempt to occupy the same niche, one species will be better at competing for limited resources and will eventually exclude the other species.  As a result, if we look at natural communities, we rarely find species whose niches overlap significantly.

Dividing Resources

Instead of competing for similar resources, species usually divide them.  For instance, the three species of North American warblers shown in the imagesall live in the same trees and feed on insects.  But one species feeds on high branches, another feeds on low branches, and another feeds in the middle.  The resources utilized by these species are similar yet different.  Therefore, each species has its own niche.  This division of resources was likely brought about by past competition among the birds.

• By causing species to divide resources, competition helps determine the number and kinds of species in a community and the niche each species occupies.

How does competition shape communities?
 

REVIEW & DO NOW Answer the following questions:
What is the competitive exclusion principle? Why does it work? What is a result of the principle?	How does the competive exclusion principle determine the number and types of species that can occupy a community?

Predation, Herbivory, and Keystone Species
How do predation and herbivory shape communities?

Virtually all animals, because they are not primary producers, must eat other organisms to obtain energy and nutrients.  Yet if a group of animals devours all available food in the area, they will no longer have anything to eat! That’s why predator-prey and herbivore-plant interactions are very important in shaping communities.

Predator-Prey Relationships

Predation
An interaction in which one animal (the predator) captures and feeds on another animal (the prey)

• Predators can affect the size of prey populations in a community and determine the places prey can live and feed.

Herbivore-Plant Relationships

Interactions between herbivores and plants, like the gorilla eating a succulent plant, are as important as interactions between predators and prey.

Herbivory An interaction in which one animal (the herbivore) feeds on producers (such as plants) Herbivores can affect both the size and distribution of plant populations in a community and determine the places that certain plants can survive and grow.

Herbivores ranging from caterpillars to gorillas can have major effects on plant survival.  For example, very dense populations of white-tailed deer are eliminating their favorite food plants from many places across the United States.

Keystone Species
 

Sometimes changes in the population of a single species, often called a keystone species, can cause dramatic changes in the structure of a community. Keystone Species A single species that is usually not abundant in a community yet exerts a strong control on the structure of that community

In the cold waters off the Pacific coast of North America, for example, sea otters devour large quantities of sea urchins.  Urchins, in turn, are herbivores.  Their favorite food is kelp, giant algae that grow in undersea “forests.”

A century ago, sea otters were nearly eliminated by hunting.  Unexpectedly, the kelp forest nearly vanished.  What happened? Without otters as predators, the sea urchin population skyrocketed.  Armies of urchins devoured kelp down to bare rock.  Without kelp to provide habitat, many other animals, including seabirds, disappeared.  Clearly, otters were a keystone species in this community.  After otters were protected as an endangered species, their population began to recover.  As otters returned, the urchin populations dropped, and kelp forests began to thrive again.  Recently, however, the otter population has been falling again, and no one knows why.

How do predation and herbivory shape communities?
 

REVIEW & DO NOW Answer the following questions:
What is predation? Give an example. How does predation affect a community? What is herbivory? Give an example.	How does herbivory affect a community? What is a keystone species? Give an example of a keystone species an how it affects its community.

Symbioses
What are the three primary ways that organisms depend on each other?

Any relationship in which two species live closely together is called symbiosis, which means “living together.” Biologists recognize three main classes of symbiotic relationships in nature: mutualism, parasitism, and commensalism.

Symbiosis
A relationship in which the individuals of two species live close together

Mutualism

The sea anemone’s sting has two functions: to capture prey and to protect the anemone from predators.  Even so, certain fish manage to snack on anemone tentacles.  The clownfish, however, is immune to anemone stings.  When threatened by a predator, clownfish seek shelter by snuggling deep into tentacles that would be deadly to most other fish, as seen in the striking image below.  But if an anemone-eating species tries to attack their living home, the spunky clownfish dart out and fiercely chase away fish many times their size.  This kind of relationship between species in which both benefit is known as mutualism.

Mutualism
Symbiotic relationship in which both species benefit from the relationship
 

Parasitism Tapeworms live in the intestines of mammals, where they absorb large amounts of their hosts’ food.  Fleas, ticks, lice, and leeches live on the bodies of mammals, feeding on their blood and skin, as seen below, or wasps who lay their eggs inside of caterpillars.  These are examples of parasitism.

Parasitism A symbiotic relationships in which one organism lives inside or on another organism and harms it The parasite obtains all or part of its nutritional needs from the host organism.  Generally, parasites weaken but do not kill their host, which is usually larger than the parasite.

Commensalism

Small marine animals called barnacles often attach themselves to a whale’s skin, as seen below.  The barnacles perform no known service to the whale, nor do they harm it.  Yet the barnacles benefit from the constant movement of water—that is full of food particles—past the swimming whale.  This is an example of commensalism.

Commensalism
A symbiotic relationship in which one organism benefits and the other is neither helped nor harmed

What are the three primary ways that organisms depend on each other?
 

REVIEW & DO NOW Answer the following questions:
What is symbiosis?	Name and define the three types of symbiosis, and give an example of each.